Voltage-fed inverters are known in the art as devices that generally receive a DC voltage source at their input and provide either a single phase or a polyphase AC voltage output. The DC voltage source is often obtained from a utility line or other AC source through a rectifier and a filter. The AC voltage output is typically a regulated AC voltage that is generally unaffected by load parameters. Such devices have a variety of uses, such as driving AC motors or providing power for AC uninterruptible power supplies (UPSs). A multi-inverter drive system is often used to provide three-phase power to a load, such as an AC induction motor.
These systems may be connected in various configurations providing different advantages. For example, single-phase inverters may be configured in either a star or a mesh configuration for collectively providing polyphase output. A three-phase star configuration, also known as a wye configuration, generally includes single-phase inverters that share a common neutral for collectively providing three-phase power. A three-phase mesh configuration, also known as a delta configuration, generally includes single-phase inverters that are each connected to two adjacent inverters to form a serial loop for collectively providing three-phase power. A polyphase inverter, which is a single inverter that provides multiple phases of power, such as a three-phase inverter, may also provide certain advantages.
Each of these configurations may provide advantages in varying situations. For example, polyphase inverters may require fewer parts and therefore be less expensive than a comparable configuration of single-phase inverters. Further, single-phase inverters in a polyphase configuration may provide more controlled output than a polyphase inverter. Additionally, for providing polyphase output, star configurations versus mesh configurations may be preferable in different circumstances. Although each of these systems as well as combinations of these systems are known, an inverter system having a particular configuration may not be optimal for driving all operational stages of a load or for all circumstances. Thus, it may be desirable for an inverter system to be able to dynamically change configurations to accommodate different operational stages and different circumstances.
One such circumstance in which a configuration change may be desirable is the loss of an inverter cell. Typically, multi-inverter drives systems are not able to function with the loss of an inverter power cell. However, single phase multi-inverter systems configured to provide polyphase output are known that can provide reduced power to a load when one of the inverter power cells becomes inoperable. For example, FIG. 1 shows a single pole multi-inverter system 10 having failure related circuitry known in the art for providing reduced power when an inverter power cell fails. The system 10 includes a high voltage three-phase power source 12, a transformer 14, six isolated single pole inverter cells SPI-UL through SPI-W2 16, and a load 18, such as a three-phase AC induction motor. Pairs of power cells, SPI-U1 and U2, SPI-V1 and V2, and SPI-W1 and W2, are each connected in series to provide each phase of output power, U, V, and W respectively.
FIG. 2 is a circuit diagram of one of the single pole inverter cells 16. Each single pole inverter cell 16 as shown is a conventional full bridge three-level inverter, which generates an AC voltage wave cycling between positive, zero and negative levels. The rectifier bridge (REC) 20 of each inverter 16 receives three-phase power from transformer 14 and converts it to DC power. C 22 is a DC voltage smoothing capacitor and GTR1 through GTR4 24 are transistors for inverting DC power to AC power. A braking circuit 26 is often added to such a conventional system 10 for dissipating excess voltage generated during deceleration of motor 18. Braking circuit 26 typically includes a braking resistor DBR and a braking transistor GTR5. FIG. 3 shows the three-phase inverter circuit for conventional system 10. By-pass circuits CTT-U1 through CTT-W2 28 may be added to system 10 in parallel with each inverter cell 16 for respectively by-passing cells as needed.
During full-power operation of system 10, by-pass circuits 28 are open and full current with full voltage is applied to load 18 as generated though matching pairs of inverter cells. When one cell, for example SPI-U1, is broken, by-pass circuits CTT-U1, CTT-V1 and CTT-W1 are closed. As such, SPI-U1, SPI-V1 and SPI-W1 are bypassed and SPI-U2, SPI-V2 and SPI-W2 collectively provide reduced three-phase power as an inverter circuit in a wye configuration. However, the resulting inverter circuit only provides half the voltage with full current to the output load compared to full operation. Thus, a load such as motor 18 could be driven continuously by inverter system 10 during failure of a cell, albeit at half or less power after failure compared with prior to failure. Further, only half of the braking torque is available after failure through SPI-U2, V2 and W2 at regeneration (braking) mode versus full braking with all six single pole inverters 28 in use along with their corresponding braking circuit 26.
Such known systems can provide continuous operation of motor 18 during failure of a power cell and can effectively provide braking torque; nonetheless, there are problems with these known systems. For instance, they may require twice as many power cells as necessary to provide three-phase power. Redundant power cells add increased expense to the system compared with a single power cell for each phase providing full voltage at full current. Additionally, during failure of a power cell, system 10 provides only half or less power to load 18 compared with full operation. Such a power reduction may be unacceptable and inefficient in many circumstances. Also, dynamic voltage sharing during transistor switching may be problematic with three-level inverter cells 16 connected in series. Such problems may be avoided with a multi-level or neutral point clamped (NPC) inverter.
Further, such a conventional system provides half or less braking torque at regeneration when one power cell is disabled, which may be inefficient and/or unsafe. Thus, mechanical braking may also be needed to assist braking when one cell is inoperable, which wastes power that could be captured during regeneration. Also, the use of individual braking circuits for each power cell may be less efficient, require more components, and be more expensive than an inverter system having a common braking circuit.
Accordingly, a need exists for a multi-inverter system that can provide greater than half of the output power during failure of a power cell without the added costs and dynamic voltage sharing problems of cell redundancy, and that can more efficiently provide braking torque during regeneration. Further, a need exists for an inverter system that can dynamically change configurations as needed to provide improved performance in various circumstances, such as the loss of power cell or for regenerative braking.